Transcript Document
Asteriods • “Minor planets”, ranging in size from several hundred km to boulders (most less than 10 km) • 104 to 105 objects (with 106 to 107 km average separations) • Orbits are fairly circular, most between Mars and Jupiter; some orbits in resonance with Jupiter; some Earth-crossing Asteroid Orbital Resonances Jupiter determines the structure of the asteroid belt, and probably is the reason a larger planet is not found there (likely promoted collisions between large protoplanets in that region). Production of Asteroids (and meteors) (Such bodies are icy in the outer solar system.) Visiting the Asteroids Gaspra Ida and Dactyl Eros and the NEAR mission Eros appears to be a solid object, apparently formed near the beginning of the solar system. It is thus a “planetesimal”. Craters large and small Impacts clearly play a role in Eros’ history. Since there is little mass and the rock is tumbling, the apparent gravity acts very strangely. Landing on an asteroid Icy Planetesimals : Comets Of similar size and shape, the comets turn out to be icy versions of the asteroids. Despite their bright appearance, they are actually much darker, covered with carbonaceous “tars”. As they approach the Sun, the ices vaporize, blowing off gas and dust and giving a kick to the comet’s orbit. Parts of a Comet The actual nucleus is tiny compared to the tails (which contain almost nothing). They always point away from the Sun. The comet does not rush through the sky although it looks like it might be doing that. Bright Comets Halley’s Comet 1986 Hyukatake 1998 Hale-Bopp 1997 Comets can be beautiful Ikeya-Seki 1965 Meteor Showers : Remnant Comets A comet leaves a trail of debris. Convergent point Some parts can be thicker than others. Some passages make “meteor storms”. The Leonid Meteor Shower November 18, 2001 Great view from the Bay Area Several bright meteors per minute Meteors and Meteorites Iron-Nickel Rocky Types of Meteors • “Carbonaceous”: lighter elements (C, Si, Mg), can be covered with dark organic compounds or rocky material – Source: unformed planet or planetesimal • “Metallic”: dominantly nickel/iron – Source: differentiated body broken up later • “Basaltic”: like lava; must have been melted rock – Source: differentiated body or large collision Many meteors are chunks from asteroids (some sources are known), others are pieces from comets Inside Meteorites Many meteors contain spherical “chondrules”. The formation of these has been mysterious, but a recent Berkeley theory shows how they might have been melted by the young Sun. The Sources of Meteors and Comets Jupiter and the other giants are responsible for the asteroids and the Oort cloud comets. The Kuiper Belt is simply the outer remnant of the solar nebula. Sometimes they hit us… Some meteors are large enough to survive passage through the atmosphere and strike the ground with enough force to be vaporized and release large amounts of energy. They hit at 50 km/s, so a rock the size of a building can make a hole a mile across. The Shoemaker-Levy Impact A comet which broke up inside Jupiter’s Roche limit also found itself on an orbit which intersected the surface… Like a machine gun burst, the fragments struck at the same latitude, one after the other... Deep Impact… It’s a good thing it wasn’t us… The Dinosaurs weren’t so lucky… It is widely accepted that the discovery of iridium in a clay layer at the end of the Cretaceous marks a mass extinction caused by the impact of a large comet (the “K-T” impact). Boom!!!! How likely is it? The end of all surface life on the Earth Too large an impactor (> 50 mi) would replace our entire atmosphere with rock vapor for a few centuries. Only life in the deepest oceans would survive. Maybe this happened near the beginning…